Two-photon probe of the Jaynes-Cummings model and controlled symmetry breaking in circuit QED

Superconducting qubits behave as artificial two-level atoms and are used to investigate fundamental quantum phenomena. In this context, the study of multiphoton excitations occupies an important role. Moreover, coupling superconducting qubits to onchip microwave resonators has given rise to the field of circuit quantum electrodynamics (QED). In contrast to quantum-optical cavity QED, circuit QED offers the tunability inherent to solid-state circuits. Here, we report on the observation of key signatures of a two-photon-driven Jaynes-Cummings model, which unveils the upconversion dynamics of a superconducting flux qubit coupled to an on-chip resonator. Our experiment and theoretical analysis show clear evidence for the coexistence of one-and two-photon-driven level anticrossings of the qubit-resonator system. This results from the controlled symmetry breaking of the system hamiltonian, causing parity to become a not-well-defined property. Our study provides fundamental insight into the interplay of multiphoton processes and symmetries in a qubit-resonator system.